Fiber coupler

ABSTRACT

A fibre coupler is provided, which includes a tubular enveloping structure and several optical fibres arranged in the enveloping structure, each of which has a fibre core and a fibre cladding surrounding same, in order to conduct laser radiation, and each of which extends from the first as far as the second end of the enveloping structure. The enveloping structure includes a tapering section which is tapered in a first direction from the first as far as the second end. In the tapering section, both a first ratio of the diameter of the fibre core to the diameter of the fibre cladding and also a second ratio of the diameter of the mode field of the laser radiation conducted in the optical fibre to the diameter of the fibre core, increases in the first direction for each optical fibre.

The present invention relates to a fibre coupler with a tubularenveloping structure and several optical fibres arranged in theenveloping structure, each of which has a fibre core and a fibrecladding surrounding same, in order to conduct laser radiation, and eachof which extends from the first as far as the second end of theenveloping structure, wherein the enveloping structure has a taperingsection which is tapered in a first direction from the first as far asthe second end.

Such fibre couplers for geometric coupling of laser radiation conductedin the optical fibres are formed for example by drawing and merging theoptical fibres. To achieve a high brilliance in the output beam bundleexiting the fibre coupler, the ratio of core diameter to overalldiameter of the optical fibres can be increased up to the output side inthe coupler, as described e.g. in EP 2 071 376 A1. Alternatively, theratio of the mode field diameter of the laser radiation conducted in theoptical fibres to the core diameter in the coupler can be increased (US2010/0278486 A1, US 2010/0189138 A1) wherein, during a tapering bydrawing the optical fibres by more than the factor of three, complexrefractive index structures are required to achieve a high brilliance.

Starting from this, the object of the invention is to provide a fibrecoupler which transfers the laser radiation conducted in the opticalfibres in an output beam bundle, wherein the brilliance of the outputbeams, which exit from the individual optical fibres and which form theoutput beam bundle, is maintained to the greatest possible extent.

The object is achieved by a fibre coupler with a tubular envelopingstructure and several optical fibres arranged in the envelopingstructure, each of which has a fibre core and a fibre claddingsurrounding same, in order to conduct laser radiation, and each of whichextends from the first as far as the second end of the envelopingstructure, wherein the enveloping structure has a tapering section whichis tapered in a first direction from the first to the second end,wherein, in the tapering section, both a first ratio of the diameter ofthe fibre core to the diameter of the fibre cladding and also a secondratio of the diameter of the mode field of the laser radiation conductedin the optical fibre to the diameter of the fibre cladding increases inthe first direction for each optical fibre.

With such a fibre coupler, the desired geometric coupling of the laserradiation (output beams) exiting the optical fibres can be achieved withthe best possible brilliance of the combined output beam bundle formedby the exiting laser radiation. Additionally, the brilliance of theoutput beams exiting from the individual optical fibres is impaired onlyinsubstantially (the laser beams have not experienced any, or only asmall, coupling among themselves), with the result that the output beamsof the output beam bundle can be addressed or used individually ifdesired. By an only insubstantial impairment is meant in particular herethat the brilliance is at least 90% or preferably at least 95% of theoriginal brilliance. The output beam bundle has a higher occupancy ofthe phase space and, on the basis of the individually addressable outputbeams, enables a targeted chronological and spatial beam formation, aslocation and beam parameters of each laser beam exiting an optical fibreare clearly defined.

In the fibre coupler according to the invention, the input beamconducted in each individual optical fibre is transferred inindividually exiting output beams, wherein the brilliance of theindividually exiting output beams largely corresponds to theindividually conducted input beams. The fibre coupler according to theinvention makes possible a targeted arrangement of the individuallyexiting output beams, wherein this arrangement in particular alsopermits the individual addressing of the individual output beams, andmakes it possible for the output beam bundle to be able to formed fromseveral individually exiting parts (the output beams exiting the opticalfibres) with an overall high brilliance.

Thus, e.g. with the fibre coupler according to the invention, an inputradiation in the form of fundamental mode radiation conducted in twoindividual fibres can be transferred in a bundle of two output beamswith largely parallel beam axes, which bundle has at the second end ofthe enveloping structure a beam waist of the fundamental mode of theindividually exiting output beams, wherein the distance between the axesis smaller than the fundamental mode diameter and the power in thisfundamental mode corresponds to at least 90% of the coupled fundamentalmode power.

The optical fibres of the fibre coupler according to the invention arepreferably fibres which are suitable also for transporting laserradiation over greater distances. The enveloping structure can have thestructure of an internal tube and a surrounding enveloping tube or canbe made of same. In particular, the fibre coupler can be formed as amonolith.

With the fibre coupler according to the invention the optical fibres arepreferably fundamental mode or low mode optical fibres, corresponding toa V parameter of between 2.4 and 4.5. For step index fibres, the Vparameter is produced from the core diameter d₁, the wavelength λ andthe numerical aperture (NA=√{square root over (n₁ ²−n₂ ²)}) wherein n₁and n₂ stand for the refractive index of the core or of the cladding:

$V = \frac{\pi \; d_{1}{NA}}{\lambda}$

Thus the optical fibres of the fibre coupler have, on the input side(thus before the tapering section), preferably a V parameter of between2.4 and 4.5, in particular of between 2.8 and 4.5. On the basis of thetapering section, the V parameter of the optical fibres of the fibrecoupler has, on its output side, preferably a value from the range of1.8 to 2.0.

For fibres with a more complex refractive index profile, an equivalent Vparameter can be used accordingly.

At a wavelength of 1.0-1.1 μm, preferably fibres are used which, beforethe enlargement of the ratio of the core diameter to the claddingdiameter and/or of mode field diameter to core diameter, have a corediameter of between 10 and 25 μm and a cladding diameter of between 125and 250 μm with mode fields of between 8 and 25 μm. Furthermore, thefibre claddings of the optical fibres preferably have an homogeneousrefractive index profile.

The optical fibres are fused with one another at the second end directlyand/or via the enveloping structure. The optical fibres are also fusedat least in the tapering section directly and/or via the envelopingstructure.

The optical fibres are not fused with one another outside of theenveloping structure before the first end.

As the optical fibres at the first end are conducted out of the fibrecoupler individually as far as the fused second end and without freebeam interface, the fibre coupler according to the invention can becalled a monolithic fibre coupler.

Preferably, the transition from non-fused to completely fused regiontakes place at a point in the fibre coupler at which the enlargement ofthe core diameter relative to the cladding diameter and enlargement ofthe mode field diameter relative to the core diameter of the opticalfibres remains still under a factor of 1.2 in respect of the untapereddiameter, with the result that the fibres still retain 80% or more oftheir original overall diameter (diameter of the quartz cladding). Thusit is achieved, advantageously, that forces occurring upon fusing causeonly such deformations in the fibre cladding and in the fibre core whichhave only an insubstantial effect on the conducting properties of theoptical fibres.

With the fibre coupler according to the invention, the thickness of thefibre cladding in the first direction can decrease in order to have theincrease of the first ratio. This can be produced easily, e.g. by“removing” the fibre claddings of the optical fibres, e.g. with the helpof an etching process. Alternatively, the refractive index profile ofthe fibre claddings can be changed, with the result that a part of thefibre cladding functions as a core. This can be achieved e.g. viathermally induced diffusion processes.

Furthermore, with the fibre coupler according to the invention, themaximum value of the second ratio can be chosen such that, in spite ofan overlapping of the (conducted) modes in adjacent optical fibres,beams are still individually conducted in each optical fibre. For thisit is ensured e.g. that there is an homogeneous refractive index beyondthe optical fibres. This is achieved by the choice of geometry andmaterial of the fibre cladding and the enveloping structure or othermaterials occupying the spaces between the fibres, with the result thatthere is a ring (seen in cross-section) with nearly homogeneousrefractive index around each core of the optical fibres used for lightconduction.

With the fibre coupler according to the invention, the diameter of thefibre core can decrease in the first direction in order to have theincrease of the second ratio. This can be achieved easily by drawing thefibre coupler. In order to guarantee the safety of the process andprevent disruptions, only the fused region should be drawn. By drawingthe optical fibres, the core becomes smaller and the mode field alsocontinues to extend in the outer region, wherein the parameters are tobe chosen such that the mode field does not extend significantly in thenext core. The geometric space for superimposing the individual laserirradiations from the individual optical fibres is thus used optimally,and it is simultaneously ensured that the output beams exiting theindividual optical fibres can still be addressed individually.

With the fibre coupler according to the invention, the envelopingstructure can have the structure of an internal tube (which e.g. isformed as a multicapillary), which internal tube comprises precisely onethrough bore for each optical fibre. With such the structure of aninternal tube the fibre coupler can be produced easily with the desiredprecision.

Furthermore, a method for producing a fibre coupler is provided in whichseveral optical fibres are provided, each of which has a fibre core anda fibre cladding surrounding same in order to conduct laser radiation,with a cladding thickness decreasing in longitudinal direction of theoptical fibres, the several optical fibres are inserted in a tubular rawenveloping structure such that they lie adjacent to one another, theoptical fibres are fused with one another directly and/or via the rawenveloping structure and the raw enveloping structure is tapered in atapering section with the inserted optical fibres in a first directionwith the result that, in the tapering section, both a first ratio of thediameter of the fibre core to the diameter of the fibre cladding andalso a second ratio of the diameter of the mode field of the laserradiation to be conducted in the optical fibre to the diameter of thefibre core increases in the first direction for each optical fibre.

With this method a fibre coupler according to the invention can beproduced with high precision in reproducible manner.

With the method according to the invention, the raw enveloping structurecan have the structure of an internal tube (e.g. a multicapillary) withbores for each optical fibre, wherein precisely one optical fibre can beinserted in each bore.

Alternatively, with the method according to the invention, the rawenveloping structure can have a tube with a bore in which the opticalfibres are inserted and then lie directly adjacent to one another.

In particular, the bore(s) can be designed tapered before the opticalfibres are inserted.

Furthermore, when fusing the optical fibres, they can be loaded withtensile stress. Furthermore, the tubular raw enveloping structure canalso be loaded with tensile stress. Additionally, the bore(s) can alsobe loaded with negative pressure. These measures help prevent undesiredbending of the optical fibres.

The production method according to the invention can also still containthe production steps described in conjunction with the fibre coupleraccording to the invention.

It is understood that the features mentioned above and those yet to beexplained in the following are applicable, not only in the givencombinations, but also in other combinations or singly, withoutdeparture from the scope of the present invention.

The invention is explained by way of example in yet greater detail inthe following with reference to the attached drawings, which alsodisclose features essential to the invention. There are shown in:

FIG. 1 a perspective view of the fibre coupler according to theinvention;

FIG. 2 a longitudinal section of the fibre coupler according to FIG. 1;

FIG. 3A a sectional view along section A-A from FIG. 2;

FIG. 3B a sectional view along section B-B from FIG. 2;

FIG. 3C a sectional view along section C-C from FIG. 2;

FIG. 4 an optical fibre with tapering cladding;

FIG. 5 a perspective representation of the tapered raw envelopingstructure with the structure of an internal tube;

FIG. 6 a longitudinal section of the raw enveloping structure with thestructure of an internal tube according to FIG. 5;

FIG. 7 a diagram in respect of the V parameter of the optical fibres ofthe fibre coupler according to the invention,

FIG. 8 a lateral view of the fibre coupler according to the invention;

FIG. 9A a sectional view along section A-A from FIG. 8;

FIG. 9B a sectional view along section B-B from FIG. 8;

FIG. 9C a sectional view along section C-C from FIG. 8, and

FIG. 10 a sectional view of a fibre coupler according to a furtherembodiment.

In the embodiment shown in FIGS. 1-3C the fibre coupler 1 according tothe invention comprises a tubular enveloping structure 2 with twoopposing ends 3, 4 as well as several (here seven) optical fibres 5,each of which has a core and a cladding arranged around same, which aretypically made of quartz glass and which fibres are arranged adjacent toone another, and extend from the first to the second end 3, 4, whereinthey pass via the first end 3 into the enveloping structure 2, as shownin particular in FIGS. 1 and 2. For better comprehension, the projectionof the fibres is reduced and the cladding has been removed in the frontpart, as well as the core, is also shown enlarged in FIGS. 3 and 4.

The tubular enveloping structure 2 comprises an internal tube structureformed as a multicapillary 6, which has a bore for each optical fibre 5,in which bore the corresponding optical fibre 5 is embedded, as well asan enveloping tube 7 surrounding the multicapillary 6. The envelopingtube 7 projects slightly beyond the multicapillary 6 at the first end 3.This projecting section of the enveloping tube 7 forms an entry region 8which surrounds the optical fibres 5 merely as a bundle and notindividually, and in which region the optical fibres 5 are not mergedwith the internal tube structure 6.

A tapering region 10 follows the entry region 8, wherein a transition 9between the two regions 8 and 10 is characterised in that, in thisregion, the connection state of the optical fibres 5 to the internaltube structure 6 changes from completely unfused to completely fused.The axial length of the transition is usually smaller than or the sameas the diameter of one of the optical fibres 5. The essential tapering,but not inevitable any tapering, of the optical fibres takes place inthis tapering region.

The tapering region 10 extends following to the transition 9 in thefirst direction as far as the second end 4 and here, the outer diameterof the optical fibres tapers in this first direction.

The optical fibres 5 can of course already also be tapered in thetransition 9. For the transition 9 it is essential that the change fromunfused to completely fused takes place here.

The optical fibres 5 each have a fibre core 11 with a preferablyrotation-symmetrical refractive index profile, and a fibre cladding 12,surrounding same, with an homogeneous refractive index. The refractiveindex profile and the geometry of the optical fibres 5 are chosen suchthat, in the section not projecting in the coupler 1 (thus the part tothe left of the coupler in FIG. 2) and within the entry region 8, theysecure a conduction of laser radiation in the fibre core 11, preferablyas fundamental mode radiation, but at most as low mode radiation.

Furthermore, the length of the optical fibres 5 to the left of the firstend 3 can be smaller or in particular larger than shown.

As can be seen in particular from the representations in FIGS. 2 and3A-3C, in the tapering region (in particular between the sections A-Aand B-B in FIG. 2) each optical fibre 5 experiences an increase in afirst ratio of the core diameter to the overall diameter. Moreover, eachoptical fibre 5 experiences an increase in a second ratio of mode fielddiameter of the laser radiation conducted in the optical fibre 5 to thecore diameter. This increase in the second ratio occurs preferably closeto the second end 4, in particular from the section B-B or from thesection C-C according to FIG. 2.

By means of this design of the fibre coupler 1 it is achieved that thelaser radiation conducted in the optical fibres 5 as input beams 13(indicated by arrows in FIGS. 1 and 2) are conducted in the opticalfibres 5 as far as the second end 4 and are in each case emitted asoutput beams and thus as a part of the output beam bundle 14, whereineach optical fibre 5 conducts the input beam 13 such that its brilliancein the output beam remains as high as possible. However, as the outputbeams of the individual optical fibres 5 are spatially packed clearlymore densely, the common output beam bundle 14 formed by the individualoutput beams can be provided, the brilliance of which bundle is clearlybetter than that of all the input beams 13 at the first end 3 of thefibre coupler 1. Furthermore, the individual output beams of the outputbeam bundle 14 are present still as separate fundamental mode beams orlow mode beams which can be advantageous for further use or employmentof the output beams, in particular in the field of materials processingwith laser radiation, such as e.g. cutting or welding of metallicworkpieces.

With the fibre coupler according to the invention 1, the optical fibres5 are fused, partly or completely, with the multicapillary 6, over thewhole tapering region 10 or at least over broadly the whole taperingrange 10.

As shown in FIG. 2, the outer diameter of the optical fibre 5 can betapered already at the transition 9 and thus the entry into themulticapillary 6. This increases the safety of the process as thecapillary diameter and the fibre diameter can be matched such that anextensive positive lock is produced and the fibre cannot move, deform orbuckle upon merging. This leads furthermore to the advantage thatdisruptions due to surface tensions, which can occur in the productionprocess during merging, can be very well avoided. Typically, the opticalfibres 5 are not yet significantly tapered at the transition 9. Amaximum tapering to not less than 80% of the original overall diameterof the respective optical fibre 5 is typical here. Moreover, theindividual optical fibres are completely fused with the multicapillaryin axial direction at a length clearly below the radius of a fibre.

The enveloping structure 2, and thus the multicapillary 6, surrounds theindividual optical fibres 5 in the fused region such that the roundcross-section of the fibre cores 11 remains intact and there is also nobending of the optical fibres 5 which significantly influences beamconduction. Furthermore, the fibre coupler 1 in the fused region isproduced such that, due to corresponding selection of the material ofthe enveloping structure 2 and in particular of the multicapillary 6 foreach individual optical fibre 5 around each core there is a region, therefractive index of which corresponds to the refractive index of thefibre cladding, the extension of which in radial direction correspondsat least to the core radius of the optical fibre 5. Additionally, thisregion is preferably highly transmissive and low scattering for theconducted radiation.

The fibre coupler 1 can for example be produced by using individualoptical fibres 5 which have been tapered in advance by eroding the fibrecladding 12, which optical fibres 5 can be used in a pre-made rawenveloping structure.

Such tapering optical fibres 5 are typically produced from opticalfibres with a constant outer diameter, such that an increasing amount ofcladding material is eroded in longitudinal direction. The claddingmaterial is preferably eroded such that the core diameter remainsconstant. This can be produced for example by a chemical erosion methodor alternatively by plasma or laser erosion. Such a tapered opticalfibre 5 is represented in FIG. 4.

Alternatively, the optical fibre can be prepared such that a diffusionof the doping between cladding and core region is caused by heating thefibre, and thus the refractive index profile is changed such that thiscorresponds to an enlarged core.

The raw enveloping structure is produced from a multicapillary (internaltube structure) with, in this case, seven parallel bores of constantdiameter, onto which the enveloping tube is fused such that the entryregion 8 is formed by the projection over the multicapillary and thestart of the multicapillary forms an end to the fusing because thecross-section of the bore has changed over a short distance. The rawenveloping structure is then tapered by thermal extraction, with theresult that there is now a raw enveloping structure 2′ shown in FIGS. 5and 6 which now has seven tapering through bores 16 which extend fromthe first end 3 as far as the second end 4. The diameter of the throughbores 16 is chosen such that it exceeds the minimum wall thicknessbetween adjacent bores 16 by at least a factor of five. Furthermore, thediameter of the through bores 16 is chosen such that one taperingoptical fibre 5 is inserted in one tapering through bore 16 and then,over the entire length of the through bore 16, the bore cross-sectionexceeds the cross-section of the optical fibre by at most a factor oftwo. In particular, the through bore 16 is, however, only slightlylarger at the end facing the entry region 8 than the inserted taperingoptical fibre 5, with the result that, after inserting the opticalfibres 5, the fusing leads to as small as possible a deformation of theoptical fibres 5. Preferably, at this end, there is a broadly positivelock between the tapering optical fibre 5 and the corresponding taperingthrough bore 16 before fusing takes place.

The fusing process is preferably carried out in longitudinal direction(axially) with as rotation-symmetrical an introduction of heat in theraw enveloping structure 2′ as possible. Such a fusing process can takeplace in a region of the raw enveloping structure 2′ to be removedlater, with inserted optical fibres 5 and thus outside of the resultingfibre coupler 1, by means of a heat source which loads the fibre couplerwith heat in the process via the later exit region (second end 4) as faras the entry region (first end 3). This causes the multicapillary tobring an end to fusing, with the result that the fibres in the entryregion 8 are not influenced. Alternatively, in the transition 9, workcan start between the unfused and fused region in the finished couplerand then the fusing can be carried out progressively in the direction ofthe exit region (second end 4) and out via here.

On the basis of the extensive positive lock between the optical fibres 5and through bores 16, with lateral supply of heat by the heating source,a deformation of the optical fibres 5 can be prevented by the occurringsurface tensions. In the first case this is at the end of the fusingprocess and with an alternative method in particular at the start of thefusing process. However, as lateral forces cannot be entirely preventedin such a transition region (between unfused and fused region), thistransition 9 is particularly advantageous at a position in which adeformation in the fibre cladding 12 has an only insubstantial effect oncore conduction. The transition 9 is preferably positioned such that thefibre diameter is still so large that no deformation of the fibre coreoccurs. For this, optical fibres 5 should be tapered to not less than80% of the original overall diameter of the respective optical fibre 5.

The fusing is carried out (irrespective of the direction when carryingout the fusing) under tensile stress applied to the raw envelopingstructure 2′ and/or the fibres 5. This is optionally still supported bynegative pressure in the through bores 16.

By means of the tensile stress, the additional tapering for the intendedincrease in the mode field diameter relative to the core diameter can beachieved already partly or entirely by drawing the coupler structurewhen fusing. However, it is preferred that the drawing is carried out onthe final geometry in a process step following the fusing. Thus therequired parameters for the output beams can be set precisely.

The possible V parameter values for the optical fibres 5 are to beexplained below in conjunction with the diagram according to FIG. 7, inwhich different parameters characterising the fibre coupler 1 arerepresented as a function of the V parameter.

The V parameter is applied along the abscissa. Curve K1 denotes the corediameter in μm of an optical fibre 5, curve K2 shows the mode fielddiameter (MFD) of laser radiation conducted in the optical fibre 5,curve K3 denotes the ratio of mode field diameter to core diameter andcurve K4 denotes the standardised intensity of the output beam decoupledfrom one of the optical fibres 5, wherein the values for thestandardised intensity are indicated along the right ordinate and thevalues for curves K1-K3 are indicated along the left ordinate. All thevalues in the diagram according to FIG. 2 are indicated for a numericalaperture of the individual optical fibres 5 of 0.07 and for a wavelengthof the coupled input beams of 1.07 μm.

On the input side the optical fibres 5 have a V value in the range offrom 2.8 to 4.5. This range is characterised in the representation inFIG. 7 as range B1. In the design of the fibre coupler according to theinvention, there is a V parameter value in the range of 1.8 to 2.0(range B2) at the second end 4, wherein this change in the V parameteris indicated in FIG. 7 by the arrow P1.

A change in the V parameter in the range B2 changes the mode fielddiameter only very slightly, with the result that drawing out the fibrecoupler 1 in the region of the second end 4 leads to a clear change inthe distances between the optical fibres 5 and thus the output beams,without the mode field diameter (without beam diameter and divergence)being substantially changed. Thus a fine adjustment of the occupancyfactor is carried out without greatly influencing the mode fielddiameter.

The range of from 1.8-2.0 for the V parameter at the second end 4 ispreferred, as values of greater than 2.0 lead to a poorer occupancy ofthe phase space and values of smaller than 1.8 to an undesired deviationof the Gaussian profile as well as a poorer reproducibility.

It is self-explanatory that basic considerations for the design,according to the invention, of the coupler, can also be transferred toother optical fibres using approximate solutions explained for stepindex fibres.

The exit region (the second end 4) is produced by shortening the rawenveloping structure 2′ fused and drawn out with the fibres by theexcess length at the end of the fusing and drawing processes.

The maximum number of optical fibres 5 and their position relative toone another can be predetermined by the corresponding design of themulticapillary 6. Therefore, the fibre coupler 1 according to theinvention can be designed with greater or fewer than the described sevenoptical fibres 5, and also deviating from the hexagonal arrangement ofthe optical fibres 5, e.g. square or annular in shape.

The intended overlap of the beam profiles (or the mode fields) of theinput beam bundle 13 is achieved by means of the region of the completefusing at the second end 4 of the fibre coupler 1, and this additionallyoffers the fibre coupler a robust and flexible interface. As there is alimited beam conduction of the individual optical fibres 5 in thisregion, beam conduction, improved in principle, is achieved by the fusedtubular enveloping structure 2 by means of the securing of the alignmentof the fibres and supporting effect.

It is also ensured that no disruption of the beam conduction induced bythe internal tube structure 6 or by fusing, in particular by deformingthe optical fibres 5, takes place within this region. This is achievedin particular in that the transition from non-, or only partially, fusedregion to completely fused region takes place in a region of good beamconduction of the individual fibres 5. This is the case in particular inthe region in which the overall diameter of the fibres 5 is not yettapered to below 80% of the original overall diameter. Thus there is asuitable adjustment of material and geometry of the optical fibres 5 andthe internal tube structure 6.

Furthermore, with the fibre coupler according to the invention 1, it isachieved that the input beams 13 conducted in the individual opticalfibres 5 do indeed overlap in the output region (and thus at the latestat the second end 4), but the individual input beams 13 are notsignificantly impaired, either by the surrounding structure or by theadjacent optical fibres. This is achieved in that the mode fieldcross-section in the output region does indeed exceed the size of thefused, individual optical fibres 5, in which region of significantintensity, however, largely the same refractive index exists as in thecladding region of the optical fibre. For this, the material of theinternal tube structure 6 is chosen such that it matches that of thefibre cladding, and a symmetrical effective refractive index profile iscreated outside of the cores, which ensures that the laser beam isconducted undisturbed mainly through the core, even if the mode fielddiameter exceeds the core diameter. In this case, the core is the regionwhich differs from the material with an homogeneous refractive index.Thus the mode can indeed have noticeable intensity in the cladding ofthe adjacent optical fibre 5, but have only insignificant intensity inthe core of the adjacent optical fibre 5. The cross-section geometry ofthe fibre core is not to be changed if possible. The extension of themodes 17 is schematically represented in the FIGS. 9A-9C, which are thesections A-A, B-B and C-C corresponding to the fibre coupler 1 shown inFIG. 8.

As an alternative to an internal tube structure 6 with a large number ofround bores for accommodating the individual optical fibres 5, theoptical fibres 5 can themselves be arranged such that the position ofthe individual fibres 5 in the coupler 1 relative to one another isdefined and makes possible an extensive positive lock. In this case, theenveloping structure 2 can have an internal tube 21 with a singlethrough bore with an adjusted cross-section, instead of themulticapillary 6.

Thus, e.g. optical fibres 5 with tapering hexagonal outer contour can bebundled in a tapering internal tube 21 which is surrounded by envelopingtube 7, as indicated in FIG. 10. Also, the start of the internal tube 21forms both a temperature barrier and the transition 9 to the fusedregion of the fibres with the enveloping structure 2 here. In therepresentation in FIG. 10, it is shown again with a dotted line that, inaddition to the optical fibre 5 used for beam conduction, which form aninternal bundle, a ring of fibres 20 surrounding this bundle is arrangedin the enveloping structure 2. These additional fibres 20 can becoreless fibres or fibres with a core (for example the same as in theinternal bundle). It needs merely to be ensured that there is asufficiently thick ring of material with a homogeneous refractive indexwhich corresponds to the cladding material of the optical fibres 5, withthe result that a symmetrical effective refractive index is producedoutside of the core, which ensures that the laser beam is conductedundisturbed, mainly through the core. The core region is, in thisinstance, the region which differs from the material with an homogeneousrefractive index. The additional material provided around the internalbundle enables an undisturbed beam conduction also with a mode fielddiameter which exceeds the diameter of the original individual opticalfibres 5. Additionally, these outer fibres 20 ensure an extensivepositive lock to the internal optical fibres 5, with the result that theinternal optical fibres 5 are not, or are only insubstantially, deformedupon fusing with the enveloping structure 2.

Furthermore, with the fibre coupler according to the invention 1,cavities may be permitted between the raw enveloping structure 2′ andthe inserted optical fibres 5 for production. In this case, opticalfibres 5 are inserted in which the glass temperature of the claddingmaterial is lower than that of the core material, with the result that,in the fusing process, the cladding material occupies the spaces and thefibre cores 11 are neither thus so deformed or so bent in cross-sectionthat beam conduction is impaired. A tensile stress on the optical fibres5 ensures that the fibre cores 11 do not suffer any fibre bending (inparticular in the transition region in which, in this variant, the lowerratio of core to overall cross-section represents an increased risk ofbending upon deformation of the cladding).

A light-conducting structure, preferably an optical fibre, can becoupled directly to the fibre coupler 1, and thus to the second end 4.As the exit surface of the second end 4 is completely fused and can thusbe prepared flexibly, the light-conducting structure can be directlycoupled to same. The optical fibre of the light-conducting structure canbe a multimode fibre which is designed such that the whole output beambundle 14 transmits in the cores of the multimode fibre, wherein themultimode fibre is preferably designed such that the brilliance of theoutput beam bundle 14 is broadly retained during conduction in themultimode fibre. This can also be a dual-core fibre, in particular onewith a fundamental mode core embedded in a multimode core, with theresult that central beams of the output beam bundle 14 are coupled inthe internal core and the remaining beams in the surrounding core.Instead of a dual core, the use of a central core with rings surroundingsame is appropriate which, in each case, can be coupled in one or morebeams. Beam conduction can then take place separately in the core andthe rings. Basically, a suitable multicore fibre can also be attached,with the result that, for example, each optical fibre 5 is assigned acore in the multicore fibre.

Alternatively, the output beam bundle 14 can be coupled via a free beampath in a light-conducting structure, in particular a fibre. The freebeam path can optionally have one or more imaging elements. Furthermore,it is possible that an imaging element is coupled directly to the secondend 4 of the fibre coupler 1.

The fibre coupler according to the invention 1 ensures that the beamaxes remain aligned and the output beams have an angular tolerance whichis smaller than its divergence angle, with the result that the outputbeams can still be addressed individually at the output or at the end 4.

The fibre coupler according to the invention 1 also ensures that theoutput beams also continue to propagate well outside of the coupler 1,thus they do not degrade.

In an alternative embodiment, the fibre coupler 1 can be provided with aso-called end-cap at the second end 4. This is made e.g. from a quartzglass or piece of fibre without a core and ensures that the output beamsdiverge. This leads to a decrease in intensity of the laser radiationand to an increase in the damage threshold of the fibre barrier layer,with the result that an anti-reflection layer or filter layer can beapplied to the exit surface of the end-cap.

1-20. (canceled)
 21. A fibre coupler, comprising: a tubular envelopingstructure; and a plurality of optical fibres arranged in the envelopingstructure, each of which includes a fibre core and a fibre claddingsurrounding same, in order to conduct laser radiation, and each of whichextends from the first as far as the second end of the envelopingstructure, wherein the enveloping structure has a tapering section whichis tapered in a first direction from the first as far as the second end,wherein, in the tapering section, both a first ratio of the diameter ofthe fibre core to the diameter of the fibre cladding and a second ratioof the diameter of the mode field of the laser radiation conducted inthe optical fibre to the diameter of the fibre core increase in thefirst direction for each optical fibre.
 22. The fibre coupler accordingto claim 21, wherein the optical fibres are completely fused with oneanother at the second end, at least one of directly and via theenveloping structure.
 23. The fibre coupler according to claim 22,wherein the transition from the unfused region to the completely fusedregion lies at a point in the fibre coupler at which the optical fibresare tapered to at most 80% of their original overall diameter.
 24. Thefibre coupler according to claim 21, wherein the optical fibres arecompletely fused with one another in the tapering section, at least oneof directly and via the enveloping structure.
 25. The fibre coupleraccording to claim 24, wherein the transition from the unfused region tothe completely fused region lies at a point in the fibre coupler atwhich the optical fibres are tapered to at most 80% of their originaloverall diameter.
 26. The fibre coupler according to claim 21, whereinthe thickness of the fibre cladding decreases in the first direction inorder to increase the first ratio.
 27. The fibre coupler according toclaim 21, wherein the maximum value of the second ratio is chosen suchthat there is an individual beam conduction in each optical fibre inspite of an overlapping of the modes in adjacent optical fibres.
 28. Thefibre coupler according to claim 21, wherein at the second end there isa homogeneous refractive index between the cores of the optical fibres,the homogeneous refractive index corresponding to the refractive indexof the cladding material.
 29. The fibre coupler according to claim 21,wherein at the second end and about each core of the optical fibresalong a cross-section surface which corresponds to the distance to thenext adjacent core there is an homogeneous refractive indexcorresponding to the cladding material of the optical fibre.
 30. Thefibre coupler according to claim 21, wherein the diameter of the fibrecore decreases in the first direction in order to increase the secondratio.
 31. The fibre coupler according to claim 21, wherein the opticalfibres are either fundamental mode or low mode optical fibres.
 32. Thefibre coupler according to claim 21, wherein the enveloping structureincludes a multicapillary, which has precisely one through bore for eachoptical fibre.
 33. A method for producing a fibre coupler, the methodcomprising: providing several optical fibres, each of which include afibre core and a fibre cladding surrounding the fibre core, the claddinghaving a thickness decreasing in a longitudinal direction of the opticalfibres; inserting the several optical fibres in a tubular raw envelopingstructure such that they lie adjacent to one another; and fusing theoptical fibres with one another at least one of directly and via the rawenveloping structure, wherein the raw enveloping structure is taperedwith the inserted optical fibres in a tapering section in a firstdirection with the result that, in the tapering section, both a firstratio of the diameter of the fibre core to the diameter of the fibrecladding and also a second ratio of the diameter of the mode field ofthe laser radiation to be conducted in the optical fibre to the diameterof the fibre core increases in the first direction increases for eachoptical fibre.
 34. The method according to claim 33, wherein the rawenveloping structure includes an internal tube structure with bores foreach optical fibre, the method further comprising inserting at least oneof the optical fibres in each of the bores.
 35. Method according toclaim 34, in which the bores are formed tapered before the opticalfibres are inserted.
 36. The method according to claim 33, wherein theraw enveloping structure includes a tube with a bore in which theoptical fibres are inserted and then lie directly adjacent to oneanother.
 37. The method according to claim 33, wherein the bore(s) areformed tapered before the optical fibres are inserted.
 38. The methodaccording to claim 33, wherein the optical fibres are charged withtensile stress when being fused.
 39. The method according to claim 33,wherein a transition from fully unfused optical fibres to opticalfibres, which are at least one of fully fused with one another and fusedvia the raw enveloping structure, is formed in axial direction.
 40. Themethod according to claim 33, wherein a second ratio of the diameter ofthe mode field of the laser radiation to be conducted in the opticalfibre to the diameter of the fibre core is increased by drawing thefused optical fibres.